1. Field of the Invention
This invention relates to an optical disc device for reading data from and writing data to an optical disc and also to a laser noise elimination circuit for reducing laser noises.
2. Description of Related Art
Optical disc devices read/write data by irradiating a laser beam emitted from a semiconductor laser onto the surface of an optical disc. However, the signal reproduced from the optical disc is adversely affected by laser noises, which are boosted by return light from the optical disc and/or temperature changes. An LNC (laser noise canceling) method is known as laser noise reduction method for offsetting the laser noise component by subtracting the monitor output signal for APC (automatic power control) from the reproduced RF signal (see, inter alia, Patent Document 1: Japanese Patent Application Laid-Open Publication No. 10-124919).
Referring to FIG. 1 of the accompanying drawings, return light of the laser beam irradiated onto an optical disc (not shown) (reflected light from the optical disc) is subjected to photoelectric conversion by a main light detector 20 and then to current/voltage conversion by a TIA (Trance Impedance Amp) 22 to become an RF signal, which is then input to an arithmetic circuit 30. On the other hand, the laser beam is partly subjected to photoelectric conversion by a front photo detector 24 for APC monitoring and then to current/voltage conversion by a TIA 26 to become an FPD (front photo detector) signal, which is then input to the arithmetic circuit 30.
Both the RF signal and the FPD signal are those obtained by photoelectric conversion of the same laser beam and hence contain a laser noise component of the same phase. Therefore, the laser noise component of the RF signal is eliminated as the FPD signal is subtracted from the RF signal by the arithmetic circuit 30 and hence the RF signal is output with a reduced laser noise.
However, while the laser noise is reduced by the above described known laser noise canceling method, the servo signal component that is constituted by a component close to DC is affected to give rise to a problem of difficulty of accurately operating the servomechanism, because a direct current (DC) component is also subtracted from the RF signal when the FPD signal is subtracted from the RF signal.
Additionally, the gain of the FPD signal that is obtained from the front photo detector 24 is defined to be low relative to the RF signal so that no saturation may take place when the optical disc device receives a laser beam of a high output power level (the larger the speed multiple for data writing, the higher the output power level) in a write mode. Therefore, the amplitude level of the FPD signal falls remarkably when the optical disc device is made to operate in a read mode and the output power level of the laser beam falls. Then, the APC operation becomes difficult and, at the same time, the laser noise contained in the FPD signal and the circuit noise of the photo detector and that of the TIA circuit and other circuits show substantially the same level, depending on the circumstances, in the laser noise eliminating operation that is conducted in a data read mode. Then, the laser noise does not decrease but may inversely increase when the FPD signal is subtracted from the RF signal. This phenomenon becomes remarkable as the speed multiple is increased for data writing.